This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to Appln. No. 2001 1243/01 filed in Switzerland on Jul. 5, 2001; the entire content of which is hereby incorporated by reference.
The invention relates to a method of fitting an impingement plate to a guide blade.
Numerous turbine blades with cooling arrangements are known from the prior art. Laid-open Specification DE 2 042 947 discloses, for example, an air-cooled blade which, in addition to an internal cooling system in a cavity, is cooled on one side by impingement cooling. Specifications U.S. Pat. No. 4,040,767, EP-A1-534207, WO99/36675 or U.S. Pat. No. 6,120,249 also show such cooling systems of turbine blades. It is generally known therefrom to cool the platform by impingement cooling. An impingement or insert plate or a cooling plate is brazed or welded onto a step of the platform and, by means of cooling holes, serves for the impingement cooling of the platform located at a certain distance below it.
The impingement plates must be tightly connected at their periphery to the platform of the blade. In one embodiment, this sealing or fastening can be produced by high-temperature brazing to a geometrically defined supporting surface on the blade. This supporting surface may be, for example, the rim of the platform or of a cavity or also existing ribs. Since these supporting surfaces or the impingement plates are often not true-to-size due to the production process, the height of the brazing gap or the quality of the brazing preparation depends on the particular person who manually carries out the tacking of the impingement plates. The impingement plates are adapted to the supporting surfaces and secured by additional weld spots. These weld spots are intended to minimize the brazing gap and prevent uncontrolled bulging during the brazing. The spot welds in particular, however, can vary in their spacing and in their contact pressure. This may lead to pronounced fluctuations in the height of the brazing gap. Since the setting of the brazing gap is a decisive process variable for the quality of the brazing, the methods used hitherto are susceptible to brazing faults.
In other cases, it has been attempted to set the brazing gap by shims or brazing foil which are placed between the impingement plate and the supporting surface. However, this process is very time-consuming and likewise requires manual work.
The object of the invention is to provide a method with which the height of the brazing gap between the impingement plate and the supporting surfaces on the platform of a guide blade can be standardized or set.
According to the invention, the object is achieved by a method of fitting an impingement plate to a platform of a guide blade of a gas turbine, the impingement plate being fastened to at least one supporting surface by means of spot welds and then being brazed onto the supporting surface to the platform by means of high temperature brazing at a brazing gap, the impingement plate having cooling holes, wherein the impingement plate is prefabricated with recesses and the spot welds are executed at the recesses.
The method according to the invention advantageously avoids a situation in which the height of the brazing gap is determined by the contact pressure during the spot welding. The depth of the recesses predetermines the height of the brazing gap.
The recesses of the impingement plate can be formed by stamping, for example at the same time as the cooling holes are made, so that no additional work is involved. The recesses should be kept as small as possible in order to avoid unnecessary losses in the brazing area. They may have, for example, a diameter of 1 to 3 mm. The height of the brazing gap may lie, for example, within a range of 0.05 to 0.1 mm.